The invention relates generally to plasma sputtering and reactors used therefor. In particular, the invention relates to the chamber shields used in plasma sputter reactors and their electrical biasing.
Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of semiconductor integrated circuits. In the most commercially important type of sputtering, DC magnetron sputtering, a metallic target composed of the material to be sputter deposited is biased negatively with respect to the chamber. Argon working gas is admitted to the chamber at a pressure in the low milliTorr range, and the electrical biasing of the target causes the argon to be excited into a plasma. The positive argon ions are attracted to the negatively biased target with sufficient energy that the argon ions sputter metal atoms from the target. Some of the sputtered atoms strike the wafer and are deposited thereon in a thin layer. In reactive sputtering, a reactive gas, for example, nitrogen or oxygen, is additionally admitted into the sputter chamber, and the sputtered metal atoms react with the nitrogen or oxygen atoms or excited radicals to form a metal nitride or oxide layer on the wafer.
One of the more demanding applications of sputtering is to deposit a metal or a metal nitride onto the sides and bottom of inter-level via holes having very high aspect ratios. That is, the holes are narrow and deep. Aspect ratios of five or more are becoming common in advanced circuit designs. Sputtering is generally a ballistic process producing a nearly isotropic emission pattern for the atoms sputtered from the target toward the wafer being sputter coated. This emission pattern is poorly suited for coating the sides of a deep, narrow hole. Assuming the continuation of the original emission pattern, relatively few sputtered atoms reach the bottom of the hole. Furthermore, for thick layers, it is likely that a sufficient number of sputtered atoms coat the upper comers of the hole to bridge the top of the hole before the bottom of the hole is filled, thereby creating a void.
Nonetheless, several approaches are available for sputtering with high aspect ratios. Several commercially important approaches increase the number of the sputtered atoms that are ionized and also cause the wafer to be negatively biased with respect to the plasma and the chamber walls. The biasing can be accomplished either by applying a negative DC bias to the pedestal electrode supporting the wafer in the target, by applying an RF bias to the pedestal electrode which, due to the dynamics of a plasma, creates a negative DC bias on the pedestal electrode, or even leaving the pedestal electrode to be electrically floating, in which case a negative DC bias still develops. The biased pedestal electrode sets up a plasma sheath adjacent to the wafer and accelerates the sputtered ions across the sheath toward the wafer with sufficient velocity that the overall pattern of the sputtered ions becomes anisotropic with a strong component normal to the wafer surface. These ions thus are oriented to reach deeply into a high aspect-ratio hole.
One method of increasing the ionization fraction involves sputtering with a high-density plasma (HDP) achieved by coupling a large amount of RF energy into the processing space through an inductive coil wrapped around the processing space. As the sputtered atoms cross the HDP processing space, they are likely to be ionized. HDP sputtering can achieve ionization fractions of above 80% and thus is very effective at deep hole coating. However, HDP sputter reactors are relatively expensive. Also, HDP sputtering is a hot process and relies on energetic ions, including argon ions produced in a working gas pressure in the range of 50 to 100 milliTorr. It is often preferred to sputter coat wafers held at relatively low temperatures, and such temperature control is difficult in an HDP sputter reactor. The quality of HDP sputtered films is not always the best.
An alternative approach, referred to as self-ionized plasma (SIP) sputtering relies upon a variety of techniques to adapt conventional DC magnetron sputter reactors to produce a relatively high plasma density near a restricted portion of the target and to extend the magnetic field lines toward the target. The ionization densities are more modest, typically around 20% though definitely above 5%, but sufficient to cause a reasonable flux deep into holes in a cool, low-pressure process using simple apparatus. SIP sputtering has been described by Fu et al. in U.S. patent application Ser. No. 09/546,798, filed Apr. 11, 2000, now U.S. Pat. No. 6,306,265 by Gopalraja et al. in U.S. patent application Ser. No. 09/518,180, filed Mar. 2, 2000, now U.S. Pat. No. 6,277,249 allowed and by Chiang et al. in U.S. patent application Ser. No. 09/414,614, filed Oct. 8, 1999. All these patent applications are incorporated herein by reference in their entireties.
Most sputter reactors, including the SIP chamber described by Chiang et al., use a grounded shield that extends over most of the chamber sidewall enclosing the processing space between the target and the pedestal. The shield extends further axially downwardly along the chamber sidewall to in back of the wafer, extends radially inwardly toward the pedestal, and then axially upwardly alongside the sides of the pedestal. The result is an annular trough having a higher outer wall and a shorter inner wall. The shield performs two functions. Sputtering inevitably coats other portions exposed inside the chamber and facing the target. When the shield becomes excessively coated, it is quickly replaced with a clean shield without the need to clean the chamber walls. Secondly, the electrically grounded shield acts as the anode in opposition to the negatively biased target cathode to support the sputtering plasma. The pedestal electrode can be held at some other potential to control the sputter deposition pattern.
Effective SIP sputtering requires relative low loss of the plasma electrons and ions to the shield. An unbalanced magnetron is one method promoted by Fu et al. in the above cited patent of reducing such loss. An unbalanced magnetron includes two magnetic poles of unequal total magnetic flux placed on the back surface of the target with the strong pole positioned near the target periphery. The asymmetric pole pattern creates not only a magnetic field component extending parallel (horizontally) and close to the front face of the target but also a vertical component extending from the target periphery away from the shield and toward the wafer before returning to the back of the stronger pole. The vertical component is sufficient to trap electrons along the field lines and thus push the plasma closer to the wafer and away from the grounded shield. Thereby, electron loss is reduced and sputtered ions are guided towards the wafer.
Chiang et al. in the above cited patent additionally include an electrically floating shield positioned between the grounded shield and the target. The floating shield collects some electrons and thus builds up a negative charge to thereafter repel further sinking of electrons. The effect is to push the plasma toward the center of the reactor.
Unbalanced magnetrons, however, suffer some drawbacks. The vertical magnetic field they produce usually extends only part ways toward the wafer before it returns to the stronger pole. Thus, its effect of guiding the plasma toward the wafer typically disappears before the wafer is reached. This shortened influence is worsened by the preference for a long-throw reactor to filter out at least part of the remaining 80% neutral flux and to improve the center-to-edge uniformity. One typical long-throw reactor has a 290 mm spacing between the target and wafer for a 200 mm wafer. Extending the magnetic field over such a distance by means of an unbalanced magnetron is difficult. Further, the asymmetric magnetic field of an unbalanced magnetron inherently degrades the uniformity of sputter coverage.
It is thus desired to provide alternatives to relying on small unbalanced magnetrons for sputter coating of deep holes. Preferably, the upper shield is grounded and the lower shield is held at a voltage higher than the plasma potential.
The biased shield is advantageously used with a sputtering apparatus and method producing a relatively high fraction of ionized sputtered atoms.